Interest in organic electronics is inspired by the promise of low-cost printed electronics and the significant scientific challenges that must be overcome for this goal to be realized. Potential applications enabled by the lower-cost processing methods and unique mechanical properties of organic electronics include lightweight flexible displays, RFID tags, and sensors. Many of these applications demand dielectrics, conductors, and semiconductors that are solution-processable under ambient conditions using conventional printing techniques. In addition to these processability requirements, important fundamental questions remain about long-range charge transport in organic solids, especially for polymeric semiconductors used in field-effect transistors (FETs).
Recent studies have demonstrated several robust, air-stable p-channel FETs using thiophene-based polymeric semiconductors. However, to the inventors' knowledge, only one family of n-type semiconducting polymers has been demonstrated with good transistor performance and good processing characteristics (vide infra). The scarcity of n-type polymeric semiconductors has limited developments in the field of organic electronics because both hole (p-type) and electron (n-type) materials and devices are required to achieve low-power consumption complementary organic circuitry (CMOS). In addition to such logic and switching applications, polymers exhibiting efficient electron transport and high electron affinities also should be useful as acceptor materials in organic photovoltaics and as electron transporting materials in polymer-based light-emitting diodes.
Limited progress has been made in developing polymer-based n-channel FETs. The first report of an n-channel FET-active polymer was benzimidazobenzophenanthroline (BBL), a ladder-type polymer processed from methane sulfonic acid. See Babel et al., J. Am. Chem. Soc., 125: 13656-13657 (2003). In that report, an electron mobility (μe) of 0.03 cm2V−1s−1 and a current on-off ratio (Ion:off) of 105 were achieved once residual acid was leached out (after annealing μe=0.1 cm2V−1s−1, Ion:off≈5). Recently, a perylene diimide-based copolymer FET using aluminum electrodes was reported to exhibit μe=1.3×10−2 cm2V−1s−1 and Ion:off=104 under inert atmosphere. See Zhan et al., J. Am. Chem. Soc., 129: 7246-7247 (2007). More recently, the inventors have demonstrated the superior performance in ambient of certain napthalene diimide-based copolymers μe>0.1 cm2V−1s−1 and Ion:off=>104). See Chen et al., J. Am. Chem. Soc., 131(1): 8-9 (2009). Other researchers achieved n-channel FET performance using poly(3-hexylthiophene) (P3HT) having a mobility of 6×10−4 cm2V−1s−1 by employing hydrophobic dielectric coatings (to minimize charge carrier trapping at the dielectric surface) and alkali metal electrodes to reduce contact resistance. However, these FETs were fabricated and characterized under inert atmosphere. See Chua et al., Nature, 434: 194-199 (2005). In addition, polymer-oligomer blends have been explored with reported performance as high as μe=0.01 cm2V−1s−1 and Ion:off>104. See Letizia et al., J. Am. Chem. Soc., 127: 13476-13477 (2005). These reports and others have advanced the limits of n-channel polymer performance via the synthesis of novel materials and/or implementation of unconventional and inert atmosphere device fabrication techniques.
It is believed that high-performance polymeric semiconductors need to maintain a delicate balance between solubility and close, efficient π-π stacking for efficient charge transport. While several p-channel polymers for OFETs meet these requirements, achieving such performance in electron transporting polymers has proven to be significantly more challenging. Previous efforts to enable n-channel polymer charge transport using environmentally-sensitive materials and contacts or hazardous acidic solvents are not compatible with the low-cost, ambient-condition solution processing requirements essential for the realization of practical organic electronics. These studies have highlighted the need for materials that can have appreciable electron mobilities, that can be soluble in conventional organic solvents, can exhibit appropriate solution rheology for printing and spin-casting, and can have a sufficiently high electron affinity to avoid electron trapping by ambient species and surface states (O2, H2O, —OH).
The realization of n-channel polymers having the aforementioned characteristics remains a significant challenge for organic electronics, because aromatic systems with appropriate electron affinity and crystallinity are typically poorly soluble, lack open positions for further substitution, and are not readily polymerizable.